A variety of in-line flowmeters are used to measure fluid flow through a pipe. In one type of flowmeter, an impeller is placed partially or entirely within the flowstream such that the force of fluid moving through the pipe turns the impeller. The rate at which the impeller turns may be measured and used to determine the fluid flow rate through the pipe. Examples of flowmeters known in the art may be found in for example, U.S. Pat. Nos. 4,433,583; 4,656,873; 5,540,107; 5,939,645; 7,603,915; and 7,913,577.
The usefulness of a given flowmeter may be judged, in one aspect, by the flowmeter turn-down ratio. Turn-down ratio is the ratio of the lowest rate of flow measurable by the flowmeter to the highest rate of flow measurable by the flowmeter. Desirably, a flowmeter will have a large turn-down ratio such that it may handle a wide range of flow rates. Known impeller-type flowmeters exhibit a turn-down ratio of about 5:1, about 12:1 or up to about 15:1. Additionally, the detection limit, i.e., the lowest rate of flow measurable by the flowmeter, may be of interest for applications in which the flowmeter is to be used to measure low flow, such as in leak detection applications.
The present disclosure relates to an impeller type flowmeter featuring a substantially improved turn-down ratio, compared to known impeller type flowmeters, and suitable for use with a pipe having a nominal inner diameter up to one inch.